Klinefelter Syndrome (47,XXY)

If you have just found out through prenatal testing that your baby has or may have Klinefelter syndrome (47,XXY) and are looking for more information, the Genetic Support Foundation is a good starting point.

There are many other important resources out there, including your doctor and genetic counselor. We have also compiled a list of resources that can provide you with additional information and support. After reviewing the information below, if you have a question that you are having difficulty finding an answer for, please feel free to contact us.

Overview of Important Information about 47,XXY (Klinefelter syndrome)

47,XXY occurs when a boy has an extra copy of an X chromosome in some or all of his cells.  It does not cause them to look much different than their peers but does cause a variety of subtle physical differences including taller than average height, lower muscle tone, and impaired fertility.  Other features that can be seen include a curved little finger, difficulty fully straightening the elbows, flat feet and a small depression in the chest.  

Individuals with 47,XXY typically have intelligence in the normal range, although some specific learning disabilities and speech/expressive language delay are common and can be supported.  Some individuals with Klinefelter syndrome have more significant health and developmental concerns, while others may have few if any notable features and not even know that they have the condition.

What Causes 47,XXY (Klinefelter syndrome)?

47,XXY occurs when there are problems with formation of a parent’s egg or sperm or when there are errors in cell division after an egg and sperm meet to form an embryo.  The exact reason for these occurrences is not known.

What are the Health and Developmental Concerns Associated with 47,XXY (Klinefelter Syndrome)?

It is important to remember that the signs and symptoms of 47,XXY can vary quite a bit from person to person.  During pregnancy, the condition may be diagnosed through genetic testing such as a chorionic villus sampling (CVS) or amniocentesis or suspected through a blood test such as cell free DNA analysis. Keep in mind that cfDNA cannot tell for certain if a baby has a variation in their number of X and Y chromosomes.

During the newborn period, boys with 47,XXY are indistinguishable from other boys.  During early childhood, low muscle tone and/or a delay in meeting developmental milestones may or may not be seen.  Between the ages of 5 and 8 they may often have an increase in height above their peers and have long arms and legs.  During school years, a lag in language skills and academic difficulties are often seen.

Many boys with 47,XXY appear to enter puberty normally but have a tendency for testosterone levels to decline in late adolescence and early adulthood.  Therefore, testosterone supplements may be given to fully develop secondary sexual characteristics such as a male muscle pattern, deep voice, and facial and body hair.

Most men with Klinefelter syndrome have infertility.  However, there have been reports of men fathering natural pregnancies and pregnancies with assisted medical technologies.  Most of these cases have occurred in men that have a mixture of 47,XXY and 46,XY cells (mosaicism).

As men with Klinefelter syndrome grow older, there is some higher risk for a range of conditions including osteoporosis, thyroid disorders, diabetes and autoimmune diseases. They also have an increased risk of acquiring leg ulcers.

If I have a baby with 47,XXY (Klinefelter syndrome), what is the chance I will have another baby with this condition?

In most cases, the chance that a woman will have another baby with an X and Y chromosome variation is thought to be less than 1% as the majority of cases of X and Y chromosome variation are not inherited.

Treatment for 47,XXY (Klinefelter syndrome):

There is no “cure” for 47,XXY as the genetic changes that cause the condition are typically present in some or all of the cells in the person’s body.

Babies with 47,XXY often do not need any special treatment or intervention as infants.  An exception is if they are born with a very small penis who will be given testosterone supplements within the first 3-4 months of life for a short time.  

If any symptoms of 47,XXY such as learning disabilities arise during childhood, they can be treated and managed with early intervention and support in school.

Many teens and adults with 47,XXY are given testosterone supplements to support full development of male secondary sex characteristics.  Testosterone also helps increase and maintain bone density but is unable to reverse infertility.  Therefore, children who are reaching adolescence should be evaluated by a pediatric endocrinologist who can determine when hormone therapy should begin.

How common is 47,XXY (Klinefelter syndrome)?

47,XXY (Klinefelter syndrome) is the most common variation in X and Y chromosomes and is estimated to occur in one of every 500 males. 


Most men with 47,XXY will be taller than average and have infertility.  Some men with Klinefelter syndrome experience difficulty in learning in some areas however, many individuals with 47,XXY have success in school, including higher education.  Men with 47,XXY work in a variety of career settings.  Some men with 47,XXY may have children on their own or with the help of assisted reproductive technologies.  Many men with Klinefelter syndrome adopt children. 

Individuals with 47,XXY will need careful monitoring of their health throughout their life but can expect to live full and productive lives.

Other Resources

Klinefelter Syndrome Information and Support: http://www.klinefeltersyndrome.org/

Klinefelter Syndrome Global Support Group www.klinefelters.org

The American Association for Klinefelter Syndrome Information and SUpport: http://www.aaksis.org/

The Association for X and Y Variations (AXYS):  https://genetic.org

The Focus Foundation: http://thefocusfoundation.org/



Bonomi M, Rochira V, Pasquali D, et al. Klinefelter syndrome (KS): genetics, clinical phenotype and hypogonadism. Journal of Endocrinological Investigation. 2017;40(2):123-134.


Groth, KA, Skakkebaek, A, Host C, Gravholt CH, Bojesen A; Klinefelter Syndrome—A Clinical Update. J Clin Endocrinol Metab 2013; 98 (1): 20-30.


National Institute of CHild Health and Human Development: https://www.nichd.nih.gov/health/topics/klinefelter/conditioninfo/Pages/symptoms.aspx


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